This invention relates to triggering of events simultaneously in a plurality of remote units, preferably by means of a radio signal sent from a control unit. The invention is particularly suitable for use in remote data acquisition systems, such as seismic data acquisition systems.
A seismic data acquisition system is disclosed in U.S. Pat. No. 4,733,744. Individual seismic source units of a surface array activated simultaneously for cumulative reinforcement may initiate seismic waves at slightly different times despite efforts at synchronisation. These variations are measured and used to compute an inverse operator which is applied to the summed uncorrected seismic recording so as to undo the distortion they produce.
A radio-link system for seismic exploration is disclosed in U.S. Pat. No. 3,062,315. The system comprises an array of geophones which transmit recordings of a seismic event using the same carrier frequency.
According to a first aspect of the present invention there is provided a method of triggering events simultaneously in a plurality of remote units, the method comprising:
transmitting a radio signal from a control unit simultaneous to a plurality of remote units; detecting reception of the radio signal at each remote unit; determining for each remote unit a compensating delay period dependent on a system delay of the radio signal; triggering an event in each remote unit upon expiry of the compensation delay period following detecting reception of the radio signal.
According to a second aspect of the present invention there is provided a system comprising: a plurality of remote units; a control unit for transmitting a radio signal to the plurality of remote units for triggering events simultaneously in the remote units; wherein each remote unit comprises detecting means for detecting reception of a radio signal sent by the control unit, means for determining a compensating delay period dependent on a system delay of the radio signal, and means for triggering an event in the remote unit upon expiry of a determined compensation delay period following detecting reception of the radio signal.
A method and system in accordance with the invention has an advantage that by including a compensating delay period the remote units are able to trigger at the same time with a greater degree of accuracy. Therefore, relative timing discrepancies that occur between the remote units are compensated for.
Preferably, the control unit transmit radio signals by means of a base station, and the remote unit receives radio signals by means of a receiver.
Preferably, the remote units are data acquisition devices, each powered by an independent power supply, such as a battery unit.
The remote units may be spatially separated from one another, and may provide an array of sensors for a telemetry system. The sensors may be suitable for seismic data acquisition, and each remote unit may include a radio transmitter for transmitting the acquired data to a receiver in the control unit.
The event triggered in a remote unit may relate to a data acquisition process, comprising a single data acquisition, or possibly a series of data acquisitions.
Suitably, the radio signal transmitted by the control unit is sent in a digital data packet comprising a predetermined labelled data sequence preceding a data field. Detection in the remote unit may then involve correlating the received labelled data sequence with an equivalent sequence stored in the remote unit.
In a seismic telemetry system, the control unit may also initiate a seismic event such as a controlled explosion, preferably via a separate radio signal, or via a control line.
The system delay, may result from one of a number of delay factors or more commonly a combination of delay factors in the system. One such delay factor may result from the radio signal being transmitted to the remote unit via a repeater. Another delay factor may occur as a result of computational delays when detecting reception of the radio signal in the remote unit. A further delay factor may result from a sample timing error in the remote unit when detecting reception of the radio signal sent as a digital data packet. In this case, data detection may occur late, resulting in a delay of up to half the sample period, or may occur early, resulting in a negative delay, that is, anticipation of the data packet by up to half the sample period.
Preferably, the sum of the compensation delay period and the propagation delay is equal to a fixed value for all the remote units. Ideally, this fixed value is equal to or greater than the maximum possible system delay relevant to that particular system.
Further features and advantages of the invention will be apparent from the description below.